It is important to ensure the clearance between the tips of each rotating turbine blade and the casing is kept to a minimum. The most efficient arrangement would have no clearance gap between the turbine blades and the casing or seal within the casing. In use, the turbine blades experience thermal expansion due to the working gases and also expand due to the rotational forces of the blades. The annular casing however is stationary and only expands due to being heated by the working gases.
Thus it is essential that there is a clearance gap between the blade and the casing to accommodate such expansion. However this gap is necessarily controlled so as to provide a minimum clearance at normal cruise conditions, whilst also ensuring that the blade tips do not rub on the shroud during transient operating conditions.
In gas turbine engines it is useful to utilise the high pressure air from the compressors to cool turbine discs and turbine blades. It is an important design consideration that the cooling air flow from the compressors is utilised to best effect. For example high pressure air from the high pressure compressor may be used to cool the combustion chamber and the high pressure turbine blade through internal cooling passages. Lower pressure air from the compressor region may be used to cool the turbine discs of the lower pressure turbine regions.